In the letter Aldrin describes the moment an alarm sounded as the astronauts were close to landing.

The craft's computer was loaded with so many tasks the alarm kept sounding - meaning the men were behind in the jobs they needed to do before they could safely land.

Auction: Aldrin is putting his papers up for sale

Aldrin's letter states: 'The lunar
landing was an experience I will always remember ... some eight minutes
into our descent engine burn, we started our most challenging part of
the landing - the approach phase sequence.

'We were behind on flight tasks due to the distractions caused by the alarms. Neil was monitoring our instruments and the visual view out his window.

'Neil's flying tasks suddenly became more complicated because the computer was sending us into a large crater containing and surrounded by boulders.

'He slowed the descent rate to just a few feet per second and studied the surrounding terrain.'Neil asked me about our fuel status and I indicated we had eight per cent remaining.

'I was then able to glance outside and began to understand why the landing sequence was taking longer than planned - the craters, rocks and boulders seemed to be everywhere.

'Mission Control radioed we had '60 seconds' of fuel remaining. Then '30 seconds' rang in our headsets.

'Neil was almost to the surface when a haze of dust was kicked up by engine exhaust. He could not see the surface and had to locate something just above the dust cloud.

Moonraker: Aldrin walking on the surface of the Moon after climbing out of the Eagle module

Man on the moon: Historic moment as Neil Armstrong's right foot leaves a print in the lunar soil

'Finally Neil was able to see a rock that appeared fixed in the stream of dust. This gave him a surface reference.

'Just as Neil placed Eagle gently on the lunar surface, I spoke the first words from the Moon: 'CONTACT LIGHT!' This was the indicator light on our control panel that told us that Eagle had touched the lunar surface.

To boldly go: The Apollo 11 mission sets off from the Kennedy Space Center in Florida

'We only had about 20 seconds of fuel remaining onboard.'

Aldrin and Armstrong were joined on the mission by Michael Collins, who orbited the Moon in the command spacecraft while they made their way to the surface.

Matthew Haley, specialist in space history at Bonhams, said: 'Apollo 11 is pretty much the high point of the American space programme and the highlight of space exploration full stop as it was the first manned lunar landing.

'These documents have been to the Moon and back are the equivalent of Christoper Columbus's logbook.

'They might appeal to an institution but this market is very much dominated by private collectors.

'The cards are pretty technical so having the letter from Buzz Aldrin with them is the icing on the cake as it helps explain just how relevant the cards were.

'The key points are the printed text about the powered ascent, when they come back from the surface of the Moon after becoming the first people to step on it.

'They would have been getting instructions from Mission Control throughout, hence the annotations.'